The present invention generally relates to storage of nuclear fuel assemblies, and more particularly to an improved spent fuel pool for wet storage of such fuel assemblies.
A spent fuel pool (sometimes, two or more) is an integral part of every nuclear power plant. At certain sites, standalone wet storage facilities have also been built to provide additional storage capacity for the excess fuel discharged by the reactors. An autonomous wet storage facility that serves one or more reactor units is sometimes referred to by the acronym AFR meaning “Away-from-Reactor.” While most countries have added to their in-plant used fuel storage capacity by building dry storage facilities, the French nuclear program has been the most notable user of AFR storage.
As its name implies, the spent fuel pool (SFP) stores the fuel irradiated in the plant's reactor in a deep pool of water. The pool is typically 40 feet deep with upright Fuel Racks positioned on its bottom slab. Under normal storage conditions, there is at least 25 feet of water cover on top of the fuel to ensure that the dose at the pool deck level is acceptably low for the plant workers. Fuel pools at most (but not all) nuclear plants are at grade level, which is desirable from the standpoint of structural capacity of the reinforced concrete structure that forms the deep pond of water. To ensure that the pool's water does not seep out through the voids and discontinuities in the pool slab or walls, fuel pools in nuclear plants built since the 1970s have always been lined with a thin single-layer stainless steel liner (typically in the range of 3/16 inch to 5/16 inch thick). The liner is made up of sheets of stainless steel (typically ASTM 240-304 or 304L) seam welded along their contiguous edges to form an impervious barrier between the pool's water and the undergirding concrete. In most cases, the welded liner seams are monitored for their integrity by locating a leak chase channel underneath them (see, e.g. FIG. 1). The leak chase channels' detection ability, however, is limited to welded regions only; the base metal area of the liner beyond the seams remains un-surveilled.
The liners have generally served reliably at most nuclear plants, but isolated cases of water seepage of pool water have been reported. Because the pool's water bears radioactive contaminants (most of it carried by the crud deposited on the fuel during its “burn” in the reactor), leaching out of the pool water to the plant's substrate, and possibly to the underground water, is evidently inimical to public health and safety. To reduce the probability of pool water reaching the ground water, the local environment and hence some AFR pools have adopted the pool-in-pool design wherein the fuel pool is enclosed by a secondary outer pool filled with clean water. In the dual-pool design, any leakage of water from the contaminated pool will occur into the outer pool, which serves as the barrier against ground water contamination. The dual pool design, however, has several unattractive aspects, viz.: (1) the structural capacity of the storage system is adversely affected by two reinforced concrete containers separated from each other except for springs and dampers that secure their spacing; (2) there is a possibility that the outer pool may leak along with the inner pool, defeating both barriers and allowing for contaminated water to reach the external environment; and (3) the dual-pool design significantly increases the cost of the storage system.
Prompted by the deficiencies in the present designs, a novel design of a spent nuclear fuel pool that would guarantee complete confinement of pool's water and monitoring of the entire liner structure including seams and base metal areas is desirable.